The present invention relates to a coding technique of a video signal, and more particularly to a predictive coding apparatus and a decoding apparatus therefor capable of compressing the amount of information by relying upon a timing correlation of the signal.
A coding method utilizing a correlation in the direction of a time axis has heretofore been well known for its high efficiency. An example may be an inter-frame coding apparatus for a television signal. According to the inter-frame coding apparatus which codes a difference between two sequential picture frames, however, the coding efficiency inevitably decreases as the motions contained in the television signal increases. To overcome this difficulty, there has been developed a motion-compensated inter-frame coding method in which the motion in the video signal is detected and accounted for in the prediction. The apparatus utilizing such motion-compensation has been disclosed in, for example, U.S. Pat. No. 4,371,895 entitled "Coded Video Signal Transmitting and Receiving System". The attention of the reader is also drawn to U.S. patent application Ser. No. 656,147 which is also related to the subject matter of the present invention.
In a predicting operation according to the motion-compensation method, a motion vector that represents a speed and a direction of the motion between two sequential frames is initially detected, and, then, a prediction signal is obtained by shifting a preceding signal in accordance with the detected motion vector so as to code a present video signal with it. When the motion vector thus detected accurately represents the actual motion of the moving portion in the video signal, a prediction error signal, associated with the coding operation, ideally becomes zero, making it possible to compress the amount of information transmitted even for a video signal which contains motion. In practice, however, the prediction error signal does not become completely zero; i.e., it often has a non-zero value near the contour of a moving portion of a television frame.
In a motionless (still) portion of a picture, the motion vector for that portion would indicate that there is no movement. The coding operation in this latter case corresponds to the so-called conventional inter-frame coding system.
The thus obtained prediction error signal and the motion vector are subjected to variable length coding prior to being transmitted. The prediction error signal does not develop much in a video signal, which contains very little movement and less complex patterns, such as is obtained during a television conference. Further, the prediction error signal has spatial (two-dimensional) correlation, which can be advantageously utilized to subject them to the variable-length coding. To date, however, the variable length coding has been conventionally effected based on using units of one scanning line at a time. For instance, to efficiently code a prediction error signal of zero value, the run-length coding had been employed, and to code the more significant prediction error signal when it is not at zero, the Huffman coding had been employed. As is known, the prediction error signal of one scanning line could be subjected to coding by combining these coding schemes.
On the other hand, the motion vector is generally detected using two-dimensional block units each spanning across a plurality of scanning lines. Often the same vector values are detected for several neighboring blocks. The difference between the motion vectors of the neighboring blocks, in many cases, therefore produce a result of zero. To date, however, the fact that zero is frequently and continuously obtained had not been taken advantage of. Consequently, an overall improved efficiency which relies upon the motion compensation had not been achieved.